KVM's work, but you could also run remote desktop software such as VNC on them all. The viewer allows you to scale down the remote screen size, so you could theoretically watch all screens from one ...
Fast and free, Chrome Remote Desktop is available on any operating system that supports the Chrome browser, including, of course, Chrome OS. It supports both unattended access and one-off screen ...
You may be used to TeamViewer or AnyDesk when helping others with PC problems, but Windows has a perfectly fine tool right there.